1. Field of the Invention
The present invention relates to a unit for use with an interferometer for measuring the planeness of thin items.
2. Description of the Prior Art
Items such as thin, stiff disks are frequently contemplated for use in environments in which opposite faces of the disks must be parallel within specified tolerances. An example of such an item is a silicon disk, used widely in various electronic applications. Quality control of such mass produced disks requires a means for determining whether such disks are within the required specified degree of parallelness or planeness. This property is measured by arbitrarily assigning a value of zero to the distance between faces at one edge of the disk, and using that reference to measure deviations therefrom along the length of one of the faces. Such deviations are known as the "wedge error".
Until recently, silicon disks with a wedge error of less than 6 .mu.m were not available. The best suctions plates for use with interferometers for measuring the wedge error of such disks had inherent wedge errors of greater than 1.5 .mu.m in the suction plates themselves. Use of such units to measure wedge errors of less than 6 .mu.m is not possible, because the wedge error inherent in the suction plate itself is a substantial percentage of the wedge error to be measured.
Another problem in the art resulting in imprecise measurement of wedge errors of less than 6 .mu.m is that conventional suction plates for holding such disks frequently result in the movement of air causing the suction plate to oscillate out of phase with the measuring apparatus, thereby resulting in incorrect interferometer readings.